1. Field of the Invention
The present invention relates to a semiconductor laser light emitting apparatus for emitting laser light using a semiconductor laser, and a solid-state laser rod pumping module for optical pumping a solid-state laser rod using semiconductor laser light so as to generate desired laser light. More particularly, it relates to a semiconductor laser light emitting apparatus for generating semiconductor laser light of high-density power with a high degree of efficiency using an array-type semiconductor laser or stack-type semiconductor laser, and a solid-state laser rod pumping module for pumping a solid-state laser rod using semiconductor laser light of high power with a high degree of efficiency so as to generate laser light having a high beam quality.
2. Description of the Prior Art
Semiconductor laser light emitting apparatuses are intended to generate combined semiconductor laser light of high power density using one or more laser light beams, each of which will be referred to as semiconductor laser light in most cases, emitted out of one or more semiconductor lasers. In order to pump a solid-state laser rod with high power and with a high degree of efficiency and then generate laser light having a high beam quality using such a semiconductor laser light emitting apparatus, it is preferable to increase the efficiency of energy utilization of the semiconductor laser light generated by the semiconductor laser light emitting apparatus. The energy utilization efficiency is directly affected by how the semiconductor laser light is focused to pump the solid-state laser rod.
In a prior art solid-state laser rod pumping module of side-pumped type intended for high power using an array-type semiconductor laser that includes a plurality of laser-light-emitting end portions integrated and stacked in the direction of a slow axis thereof, and that is placed so that the slow axis of the semiconductor laser is parallel to the axis of the solid-state laser rod, the solid-state laser rod cannot be pumped with a high degree of efficiency because of a small cross-sectional area of the solid-state laser rod in the case that the semiconductor laser light emitted out of the semiconductor laser passes through the solid-state laser rod only once. In general, such a semiconductor laser emits laser light whose near-field pattern is an ellipse according to its characteristics, the laser light having different divergence angles with respect to two axes perpendicular to the optical axis thereof (i.e., in two perpendicular longitudinal sections). The laser light has a smaller divergence angle with respect to the direction of one of the two axes, which will be referred to as slow axis, and has a larger divergence angle with respect to the direction of the other one of the two axes, which will be referred to as fast axis. To solve the above problem, a method of pumping the solid-state laser rod with a high degree of efficiency has been proposed, the method comprising the steps of injecting the semiconductor laser light into a reflection tube surrounding the solid-state laser rod, and trapping the semiconductor laser light within the reflection tube so that it passes through the solid-state laser rod a number of times.
In the case that the efficiency of trapping the semiconductor laser light within the reflection tube is high, the prior art method offers the advantage of being able to increase the efficiency of the optical pumping of the solid-state laser rod. However, in order to increase the efficiency of trapping the semiconductor laser light within the reflection tube, it is necessary to decrease the amount of light that can escape from the interior of the reflection tube via an injection hole formed in the wall of the reflection tube, through which the semiconductor laser light has been injected into the reflection tube, that is, to minimize the size of the injection hole.
In general, in order to generate laser light of high power, a stack-type semiconductor laser is used as the semiconductor laser for optical pumping the solid-state laser rod. A stack-type semiconductor laser includes a plurality of bar-shaped (or rectangular) components (or arrays) that are stacked in the direction of the fast axis of the semiconductor laser, each of the plurality of bar-shaped arrays including a plurality of laser-light-emitting end portions that are aligned and integrated in the direction of the slow axis.
In order to generate laser light having a high-beam quality by optical pumping a solid-state laser rod, there is a need to reduce the wave aberration caused by the solid-state laser rod itself because of heat generated by the optical pumping of the solid-state laser rod. To that end, it is desirable that the solid-state laser rod be an ideal grated refractive index lens, by illuminating the solid-state laser rod with semiconductor laser light as uniformly as possible, making the distribution of the light intensity of the semiconductor laser light incident on the solid-state laser rod axisymmetric with respect to the axis of the solid-state laser rod and uniform, and making the radial distribution of temperature in the solid-state laser rod second-order axisymmetric with respect to the axis of the solid-state laser rod.
However, in such a prior art solid-state laser rod pumping module, the radial distribution of temperature caused by the optical pumping of the solid-state laser rod using semiconductor laser light cannot be a second-order axisymmetric one because the semiconductor laser light is injected into only a part of the solid-state laser rod when it enters the reflection tube first after it has passed through the injection hole, i.e. part of the solid-state laser rod that is opposite to (or facing) the injection hole, without its laser intensity being reduced. Therefore, although the prior art solid-state laser rod pumping module is capable of generating laser light having average laser power up to about 1 kW if the beam quality does not matter, it can only generate laser light having average laser power of the order, at most, of about 100 Watts in the case that a high beam quality is needed.
Referring now to FIG. 19, there is illustrated a cross-sectional view showing a prior art solid-state laser rod pumping module as shown in, for example, S. Fujikawa et al., “High-power high-efficient diode-side-pumped Nd: YAG laser”, technical digest of Advanced Solid-State Lasers '97, pp. 296–299, 1997. In the figure, reference numeral 101 denotes a solid-state laser rod pumping module, 102 denotes a solid-state laser rod, and 103 denotes a cooling sleeve that is shaped like a tube and is transparent to semiconductor laser light. The cooling sleeve 103 is disposed on substantially the same axis as the solid-state laser rod 102 so that it surrounds the solid-state laser rod 102. In the cooling sleeve 103, a coolant for the solid-state laser rod 102 is circulated. In addition, reference numeral 104 denotes a diffusive reflection tube for diffusively reflecting semiconductor laser light incident thereon, which is disposed on substantially the same axis as the solid-state laser rod 102 so that it surrounds the solid-state laser rod 102 and the cooling sleeve 103, the diffusive reflection tube 104 being able to trap the semiconductor laser light injected into the interior thereof, 105 denotes a semiconductor laser having a plurality of laser-light-emitting end portions that are aligned and integrated in a direction parallel to the axis of the solid-state laser rod 102. In the example shown, the solid-state laser rod 102 is located so that its axis is parallel to the slow axis of the semiconductor laser 105. The semiconductor laser light emitting apparatus included in the solid-state laser rod pumping module consists of the semiconductor laser 105 only. Furthermore, reference numeral 107 denotes a semiconductor laser light guiding component constructed of a sheet glass, which is inserted into the diffusive reflection tube 104.
Laser light emitted out of the semiconductor laser 105 is guided towards the solid-state laser rod 102 located within the diffusive reflection tube 104 while it is totally reflected off the upper and lower surfaces of the sheet glass, which is disposed as the semiconductor laser light guiding component 107. The semiconductor laser light injected into the interior of the diffusive reflection tube 104 then enters the solid-state laser rod 102 and is partially absorbed by the solid-state laser rod 102. The remainder of the semiconductor laser light passing through the solid-state laser rod 102 is diffusively reflected off the inner surface of the diffusive reflection tube 104, so that the diffusively reflected semiconductor laser light can be uniformly distributed within the diffusive reflection tube 104, as indicated by dashed lines shown in FIG. 19. The solid-state laser rod 102 is thus illuminated uniformly by the semiconductor laser light uniformly distributed within the diffusive reflection tube 104. Heat generated in the solid-state laser rod 102 is eliminated from the outer surface of the solid-state laser rod 102 by the coolant circulated in the cooling sleeve 103.
The solid-state laser rod pumping module 101 as shown in FIG. 19 can pump the solid-state laser rod 102 with a high degree of efficiency because the semiconductor laser light guiding component 107 constructed of the sheet glass occupies only a small part of the inner surface of the diffusive reflection tube 104 whose area is relatively small and the amount of semiconductor laser light that can escape from the interior of the diffusive reflection tube 104 is therefore small.
Referring next to FIGS. 20 and 21, there are illustrated diagrams showing a prior art semiconductor laser light emitting apparatus and another prior art solid-state laser rod pumping module including the semiconductor laser light emitting apparatus, respectively, as shown in, for example, H. Bruesselbach et al., “High-Power Side-Diode-Pumped Yb:YAG Laser”, technical digest of Advanced Solid-State Lasers '97, pp. 285–287, 1997. FIG. 20 is a cross-sectional view of the semiconductor laser light emitting apparatus, the view being taken on the plane of the fast axis of a stack-type semiconductor laser included in the apparatus. FIG. 21 is a cross-sectional view of the solid-state laser rod pumping module including the semiconductor laser light emitting apparatus of FIG. 20, the view being taken on the plane perpendicular to the axis of a solid-state laser rod included in the solid-state laser rod pumping module. In the figure, reference numeral 111 denotes the solid-state laser rod pumping module, 112 denotes the solid-state laser rod, and 113 denotes a cooling sleeve that is shaped like a tube and is transparent to semiconductor laser light. The cooling sleeve 113 is disposed on substantially the same axis as the solid-state laser rod 112 so that it surrounds the solid-state laser rod 112. In the cooling sleeve 113, a coolant for the solid-state laser rod 112 is circulated. In addition, reference numeral 114 denotes a mirror-reflective reflection tube that is so shaped as to mirror-reflect semiconductor laser light injected into the interior thereof, which is disposed on substantially the same axis as the solid-state laser rod 112 so that it surrounds the solid-state laser rod 112 and the cooling sleeve 113.
Reference numeral 121 denotes the stack-type semiconductor laser including a plurality of bar-shaped components (or arrays) 121-1 to 121-5 that are stacked in the direction of the fast axis thereof, each of the plurality of bar-shaped components 121-1 to 121-5 having a plurality of laser-light-emitting end portions that are aligned and integrated in the direction of the slow axis of the stack-type semiconductor laser, 122 denotes a semiconductor laser light focusing component for focusing laser beams emitted out of the stack-type semiconductor laser 121 to an injection hole, and 122-1 to 122-5 denote cylindrical lenses that are opposite to the respective bar-shaped components 121-1 to 121-5 of the stack-type semiconductor laser 121 and that are located at a distance, which is substantially equal to the focal length of the cylindrical lenses, from the respective bar-shaped components 121-1 to 121-5. Each of the plurality of cylindrical lenses 122-1 to 122-5 serves to collimate the laser beam emitted out of each of the plurality of bar-shaped components 121-1 to 121-5 of the stack-type semiconductor laser. Furthermore, reference numeral 122a denotes a cylindrical lens array comprised of the plurality of cylindrical lenses 122-1 to 122-5 that are stacked in a direction parallel to the direction in which the plurality of bar-shaped components 121-1 to 121-5 are stacked, at the same intervals as the plurality of bar-shaped components 121-1 to 121-5 stacked, and 122b denotes a focusing lens lot for focusing the plurality of semiconductor laser beams collimated by the plurality of cylindrical lenses 122-1 to 122-5 with respect to a direction parallel to the direction in which the plurality of bar-shaped components 121-1 to 121-5 are stacked (i.e., the direction of the fast axis), to a linear cross section having a longer side running in the same direction as the axis of the solid-state laser rod 112. The semiconductor laser light emitting apparatus consists of the stack-type semiconductor laser 121, cylindrical lens array 122a, and focusing lens lot 122b. 
Reference numeral 117 denotes a semiconductor laser light guiding component formed on the mirror reflection tube 114, for guiding the semiconductor laser light, which has been emitted out of the semiconductor laser apparatus comprised of the stack-type semiconductor laser 121, cylindrical lens array 122a, and focusing lens lot 122b, into the interior of the mirror-reflective reflection tube 114. The mirror-reflective reflection tube 114 is constructed of a coating of high reflectivity formed on the outer surface of the cooling sleeve 113. On the other hand, the semiconductor laser light guiding component 117 is constructed of an anti-reflection coating shaped like a slit and formed on the outer surface of the cooling sleeve 113. The outer surface of the cooling sleeve 113 is thus covered by both the high-reflective coating that serves as the mirror-reflective reflection tube 114 and the slit-shaped anti-reflection coating that serves as the semiconductor laser light guiding component 117.
The laser light emitted out of each of the plurality of bar-shaped components 121-1 to 121-5 has a divergence angle of about 10 degrees with respect to the direction of the slow axis (i.e., in a longitudinal section of the laser light including the slow axis), and a divergence angle of about 30 to 50 degrees with respect to the direction of the fast axis (i.e., in a longitudinal section of the laser light including the fast axis). Each of the plurality of cylindrical lenses 122-1 to 122-5, which is opposite to each of the plurality of bar-shaped components 121-1 to 121-5, mainly collimates a component of the laser light emitted out of each of the plurality of bar-shaped components 121-1 to 121-5, which is diverging with respect to the direction of the fast axis. The focusing lens lot 122b can focus the collimated semiconductor laser light to a linear cross section at the focal point thereof. The focused semiconductor laser light is then injected into the interior of the mirror-reflective reflection tube 114 by way of the semiconductor laser light guiding component 117 constructed from the anti-reflection coating located in the vicinity of the focal point where the collimated semiconductor laser light is focused. The incident semiconductor laser light then enters the solid-state laser rod 112 and is partially absorbed by the solid-state laser rod 112. The remainder of the semiconductor laser light, which has not been absorbed by the solid-state laser rod 112, is reflected off the high-reflective coating of the mirror-reflective reflection tube 114, and it then enters the solid-state laser rod 112 again and is partially absorbed by the solid-state laser rod 112.
A problem with the prior art solid-state laser rod pumping module as shown in FIG. 19 is that the semiconductor laser 105 cannot pump the solid-state laser rod 102 with a large amount of electrical power, because laser light emitted out of the semiconductor laser 105 including the plurality of laser-light-emitting end portions stacked in the direction of the slow axis has to enter the solid-state laser rod 102 by way of the semiconductor laser light guiding component 107 with a small cross-section. Thus, the prior art solid-state laser rod pumping module as shown in FIG. 19 is not suitable for improvements in the output power of the solid-state laser.
The semiconductor laser light, which has been guided into the interior of the diffusive reflection tube 104 by the semiconductor laser light guiding component 107 constructed of the thin plate glass, enters the solid-state laser rod 102 first. That is, the semiconductor laser light enters the part of the solid-state laser rod 102 that is opposite to the output surface of the semiconductor laser light guiding component 107, without its laser intensity being reduced. As a result, the laser-light intensity distribution of the semiconductor laser light within the solid-state laser rod 102 has a peak on the side of the rod facing the semiconductor laser light guiding component 107. Another problem with the prior art laser rod pumping module as shown in FIG. 19 is thus that the laser-light intensity distribution of the semiconductor laser light within the solid-state laser rod 102 is not axisymmetric with respect to the axis of the rod and is not uniform, and therefore the radial distribution of temperature in the solid-state laser rod 102, which is caused by the absorption of the semiconductor laser light, deviates from a second-order axisymmetric one. As a result, the solid-state laser rod 102 serves as a grated refractive index lens having wave aberration.
A problem with the other prior art solid-state laser rod pumping module, as shown in FIGS. 20 and 21, including the prior art semiconductor laser is that it is clear from the viewpoint of the geometry of the module that part of the incident semiconductor laser light, which has not entered the solid-state laser rod 102 immediately after it was guided into the interior of the mirror reflection tube 114 by way of the semiconductor laser light guiding component 117, cannot enter the solid-state laser rod 112 even if it is reflected off the mirror reflection tube 114 a number of times, as shown in FIG. 22. The solid line of FIG. 22 designates such a ray of light that cannot enter the solid-state laser rod 112. In order to cause the semiconductor laser light to be absorbed with a high degree of efficiency by the solid-state laser rod 2, there is a need to vary the focal length of the focusing lens lot 122b so that the converging angle of the focusing lens lot 122b falls within an angle that is formed by two lines connecting both edges of the solid-state laser rod 112 with the semiconductor laser light guiding component 117 when viewed from the semiconductor laser light guiding component 117 in one plane perpendicular to the axis of the solid-state laser rod 112. On the other hand, the transverse size of the focused semiconductor laser light beam at the focal point of the focusing lens lot 122b is uniquely determined by the focal length of the focusing lens lot 122b. The transverse size is the length of one side of a cross section of the semiconductor laser light beam at the focal point of the focusing lens lot, the side running in a direction parallel to the direction in which the plurality of bar-shaped components 121-1 to 121-5 are stacked (i.e., in a direction orthogonal to the axis of the solid-state laser rod 112, or in the direction of the fast axis). Therefore, the transverse size of the semiconductor laser light beam at the focal point of the focusing lens lot 122b cannot be a minimum one obtained by means of the focusing lens lot 122b, thereby reducing the efficiency of energy utilization of the semiconductor laser light. This results in the need for an increase in the size of the semiconductor laser light guiding component 117. Another problem with the other prior art solid-state laser rod pumping module is thus that the efficiency of trapping the semiconductor laser light within the mirror-reflective reflection tube 114 is reduced.
The transverse size of the focused semiconductor laser light beam at the focal point of the focusing lens lot 122b can be theoretically calculated using (d1*f2/f1), where d1 is the transverse size of the semiconductor laser light beam at the light-emitting end portion of each bar-shaped component of the stack-type semiconductor laser, f1 is the focal length of each of the plurality of cylindrical lenses 122-1 to 122-5 of the cylindrical lens array 122a, and f2 is the focal length of the focusing lens lot 122b. Actually, the focused semiconductor laser light beam has a larger transverse size because of variations in the intervals at which the plurality of bar-shaped components 121-1 to 121-5 are stacked, pitch errors in the plurality of cylindrical lenses 122-1 to 122-5 of the cylindrical lens array 122a, and a variation in the position of the cylindrical lens array 122a. 
If the size of the semiconductor laser light guiding component 117 is reduced in order to improve the efficiency of trapping the semiconductor laser light within the mirror-reflective reflection tube 114, the transverse size of the focused semiconductor laser light beam at the focal point of the focusing lens lot 116b can become larger than the size of the semiconductor laser light guiding component 117. Another problem with the other prior art solid-state laser rod pumping module is thus that the ratio of the amount of semiconductor laser light guided into the interior of the mirror-reflective reflection rube 114 to the total amount of the semiconductor laser light focused onto the semiconductor laser light guiding component 117 is reduced, and this results in reducing the ratio of the amount of semiconductor laser light absorbed by the solid-state laser rod 112 to the total amount of the semiconductor laser light focused onto the semiconductor laser light guiding component 117. In the other prior art solid-state laser rod pumping module as shown in FIGS. 20 and 21, the ratio of the amount of semiconductor laser light absorbed by the solid-state laser rod 112 to the total amount of the semiconductor laser light emerging from the cylindrical lens array 122a is as small as 26%.
The semiconductor laser light guided into the interior of the mirror-reflective reflection tube 114 by way of the semiconductor laser light guiding component 117 enters the solid-state laser rod 112 first. In the case that the solid-state laser rod 112 has a large absorption coefficient, the intensity of semiconductor laser light reflected off the mirror-reflective reflection tube 114 becomes much less than that of the semiconductor laser light incident on part of the solid-state laser rod 112, which is opposite to the output surface of the semiconductor laser light guiding component 117. As a result, the laser-light intensity distribution of the semiconductor laser light within the solid-state laser rod 112 has a peak on the side of the rod facing the semiconductor laser light guiding component 117. Another problem is thus that the laser-light intensity distribution of the semiconductor laser light within the solid-state laser rod 112 is not axisymmetric with respect to the axis of the rod and is not uniform, and therefore, the radial distribution of temperature in the solid-state laser rod 112, which is caused by the absorption of the semiconductor laser light, deviates from a second-order axisymmetric one. As a result, the solid-state laser rod 112 serves as a grated refractive index lens having wave aberration.
In addition, since the stack-type semiconductor laser for use in the other prior art solid-state laser rod pumping module as shown in FIGS. 20 and 21 is constructed of the plurality of bar-shaped components that are stacked in the direction of the fast axis, each of the plurality of bar-shaped components including a plurality of laser-light-emitting end portions aligned and integrated in the direction of the slow axis, laser light emitted out of the stack-type semiconductor laser light has a large divergence angle with respect to a direction perpendicular to the axis of the solid-state laser rod (i.e., the direction of the fast axis), and a small divergence angle with respect to a direction parallel to the axis of the solid-state laser rod (i.e., the direction of the slow axis). Another problem is thus that although the semiconductor laser light entering the interior of the mirror-reflective reflection tube diverges largely with respect to a direction perpendicular to the axis of the solid-state laser rod, all rays of the semiconductor laser light having a small divergence angle with respect to a direction parallel to the axis of the solid-state laser rod are concentrated onto part of the solid-state laser rod which is opposite to the output surface of the semiconductor laser light guiding component 117.
Furthermore, another problem with the other prior art solid-state laser rod pumping module, as shown in FIGS. 20 and 21, including the prior art semiconductor laser, is that when the semiconductor laser light focusing component includes an aspherical lens instead of the focusing lens let 122b, the laser light beam emerging from the aspherical lens can diverge and then has a larger transverse size than the aspherical lens, according to the transverse size of the semiconductor laser light beam in the direction of the slow axis, at a distance from the semiconductor laser.